1. Field of the Invention
The present invention generally relates to high efficiency electrical power supply voltage regulators and, more particularly to improving the efficiency of voltage regulators which must supply power to highly variable loads with extremely wide variation in current requirements.
2. Description of the Prior Art
All electrically operated devices require electrical power and are designed to operate on the type of power which can be provided from the source which is most convenient in view of the intended function. Devices which are generally operated in a fixed location, such as household appliances and other devices having moderate power requirements, are generally designed and constructed to operate on efficiently transmitted alternating current power of a standard voltage while larger power requirements may require multi-phase alternating current power at higher voltages. On the other hand, devices which must be portable for their intended use generally are designed and constructed to be operated on direct current from batteries at a nominally constant voltage.
However, the amount of power which may be stored in and recovered from batteries is necessarily limited, particularly where the size or weight of the batteries must be limited for convenience of the use of the device. Moreover, as a battery is discharged, the voltage obtainable therefrom necessarily varies and decreases as the battery becomes more discharged. The internal resistance of batteries, while low in modern designs, is not negligible and also causes voltage reduction with increased load. While some devices operating on battery power may be tolerant of voltage variation, modern electronic devices using high density integrated circuits, such as may be used in so-called laptop and palm-top computers and personal digital assistants (PDAs) which have recently become popular, increasingly require extremely stable and substantially constant voltage within a tolerance of a few tenths or hundredths of a volt and thus require high quality voltage regulation.
Unfortunately, circuits capable of regulating voltage, even with relatively wide tolerances, necessarily consume a finite amount of power since the output voltage must necessarily be reduced from a higher voltage by causing a voltage drop across some components in the voltage regulator while a current is being supplied. The power consumed is thus, at a minimum, the product of the voltage drop and the current for analog regulators although such power consumption may be reduced somewhat by switching regulators as will be discussed below. If the load is relatively constant, the voltage regulator can be carefully designed to operate with a very low voltage drop and thus may be relatively efficient. However, transient changes in load current may cause corresponding fluctuations in the regulator output voltage unless the voltage is adequately filtered, generally requiring a relatively large storage capacitor or voltage regulation from a higher voltage with a correspondingly larger voltage drop so that peak currents can be supplied from the voltage regulator or a combination of both; either of which necessarily requires features which are generally undesirable in a portable device (e.g. the size and weight of filter capacitors and the increased inefficiency of the voltage regulator coupled with increased battery size and weight to compensate for that inefficiency). Further, power consumed by the voltage regulator must be dissipated as heat in the portable device and the minimum size and weight of the regulator is generally increased by both the current which must be delivered and the heat which must be dissipated. Conversely, for a given voltage regulator and filter and/or battery size and weight, the efficiency of any voltage regulator is necessarily reduced in accordance with the magnitude of changes and frequency of transients in load current it must accommodate and the accuracy of voltage regulation which must be provided.
These interrelated problems are particularly acute in regard to portable data processing devices such as laptop computers and similar device alluded to above. The duration of operation for each use cycle is generally a significant fraction of an hour, at a minimum, while digital processing, memory and logic circuits required therein require extremely close tolerances of voltage regulation, size and weight constraints are severe for commercially competitive designs and, most importantly, the changes in load current are particularly large, especially in modern processor designs with sophisticated power saving circuits. More specifically, modern processors are generally designed to enter one of a plurality of “sleep states” relatively quickly when an operation is completed and no new data or command is entered. Thus, while the peak power requirements of the processor and associated circuitry may be, for example, 50 Watts, the average power consumed is a small fraction of that requirement, for example, an average power consumption of 5 Watts or less. The duty cycle of the peak power consumption may be substantially less than ten percent. Much the same scenario is presented by the display which generally consumes far more power than the processor but which may be blanked after a relatively short period during which the display is unchanged.
Thus, in general and on average, the display represents about 33% of the power load, the processor represents about 10% of the power load while other associated devices such as a hard disk storage, clock, memory, modem, network interfaces and the like, some of which may be intermittent loads, represent slightly less than half of the power load. Thus, at the present state of the art, the voltage regulator may consume an amount of power comparable to that required, on average, by the processor and is thus a significant factor in battery life and a significant limitation on the period of usability of the laptop computer or other portable digital device per battery charge while the efficiency of the voltage regulator is generally comparatively lower than for many other devices in view of the close regulation required and the wide variation in loads which must be accommodated.